1. Field of the Invention
The present invention relates to a lens barrel comprising a cylindrical cam provided with a spiral cam slot for moving a photographing lens system in focusing mode or in zooming mode.
2. Related Background Art
The above-mentioned type of the lens barrel generally contains two or more photographing lens systems disposed therein and performs focusing or zooming by changing the distance between the lens systems.
In the construction for focusing or zooming hitherto generally known, the lens barrel comprises a guide tube in which a member for holding the photographing lens system, for example, a lens frame is mounted. On a part of the guide tube there is formed a linear slot in which a cam pin is engaged. Through the cam pin, the lens frame is supported for movement along the optical axis. Rotatably relative to the guide tube, there is mounted a cylindrical tube on which a spiral cam slot is formed. The spiral cam slot extends circumferentially and inclined to the optical axis. The above-mentioned cam pin is engaged in the spiral cam slot. With the rotation of the cylindrical cam, a force intending to move the lens system forwardly and backwardly in the direction along the optical axis can be transmitted to the lens frame through the cam pin.
The spiral cam slot on the cylindrical cam plays an important role regarding the function to give a lift to the lens system in the direction along the optical axis. For the prior art cylindrical cams made of resin by molding, various methods have been employed to form such spiral cam slot on the cylindrical cam. Some examples thereof are shown in FIGS. 8 through 10(B).
In the example shown in FIG. 8, the cylindrical cam 1 has a cam 2 formed by single surface only.
In another example shown in FIGS. 9(A) through 9(E), the spiral cam slot has two side surfaces. One of the two side surfaces is formed by a surface perpendicular to the axis of the cylindrical tube and the other is formed by a surface so inclined as to widen the slot toward the open side.
In a further example shown in FIGS. 10(A) and 10(B), a spiral cam slot 2 is formed on the inside surface of the cylindrical cam 1. The cross-section of the cam slot has a V-form. Engaged in the cam slot is a cam pin 4 which is radially biased by a spring 5 on the side of the lens frame 3.